A Dual Connectivity (DC) framework is currently being considered for Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) Release 12 (Rel-12). DC refers to the operation where, while in a connected state (RRC_CONNECTED), a given User Equipment (UE) consumes radio resources provided by at least two different network points, namely a Master enhanced or evolved Node B (eNB) (MeNB) and Secondary eNB (SeNB), connected via a non-ideal backhaul connection. A UE in DC maintains simultaneous connections to an “anchor node” and a “booster node,” where the MeNB is interchangeably referred to herein as the anchor node and the SeNB is interchangeably referred to herein as the booster node. As the name implies, the MeNB controls the connection and handover of the SeNB. No SeNB standalone handover is defined for LTE Rel-12. Signaling in the MeNB is needed even in SeNB change. Both the anchor node and the booster node can terminate the control plane connection towards the UE and can thus be the controlling nodes of the UE.
The UE reads system information from the anchor node. In addition to the anchor node, the UE may be connected to one or several booster nodes for added user plane support. The MeNB and the SeNB are connected via the Xn interface, which is currently selected to be the same as the X2 interface between two eNBs.
More specifically, DC is a mode of operation of a UE in RRC_CONNECTED state in which the UE is configured with a Master Cell Group (MCG) and a Secondary Cell Group (SCG). A Cell Group (CG) is a group of serving cells associated with either the MeNB or the SeNB, respectively. The MCG and the SCG are defined as follows:                The MCG is a group of serving cells associated with the MeNB, comprising of a Primary Cell (PCell) and optionally one or more Secondary Cells (SCells).        The SCG is a group of serving cells associated with the SeNB comprising of a Primary SCell (pSCell) and optionally one or more SCells.The MeNB is the eNB which terminates at least S1 Mobility Management Entity (MME). The SeNB is the eNB that is providing additional radio resources for the UE but is not the MeNB.        
The anchor node (MeNB) and the booster node (SeNB) roles are defined from a UE point of view, which means that a node that acts as an anchor node to one UE may act as booster node to another UE. Similarly, though the UE reads the system information from the anchor node, a node acting as a booster node to one UE may or may not distribute system information to another UE. The MeNB provides system information, terminates the control plane, and can terminate the user plane. The SeNB can terminate the control plane and does terminate the user plane. In some cases, the SeNB terminates only the user plane.
In one application, DC allows a UE to be connected to two network nodes to receive data from both network nodes to increase its data rate. This user plane aggregation achieves similar benefits as Carrier Aggregation (CA), but uses network nodes that are not connected by a low-latency backhaul/network connection. Due to this lack of low-latency backhaul, the scheduling and Hybrid Automatic Repeat Request (HARQ) Acknowledgement (ACK) feedback from the UE to each of the network nodes will need to be performed separately. That is, it is expected the UE shall have two uplink transmitters to transmit uplink control and data to the connected network nodes. In a typical scenario, the dual links with the MeNB and the SeNB belong to different carrier frequencies and even different frequency bands.
Since DC operation involves two non-co-located transmitters (i.e., the MeNB and the SeNB), one of the main issues related to UE receiver performance is the maximum receive timing difference (Δt) of the signals from the MeNB and the SeNB received at the UE receiver. This gives rise to two cases of DC operation with respect to the UE: synchronized DC operation and unsynchronized DC operation.
As used herein, synchronized DC operation means that the UE can perform DC operation provided the receive timing difference (Δt) between the signals received at the UE from the Component Carriers (CCs) belonging to the MCG and the SCG are within a certain threshold, e.g. ±33 microseconds (μs). As a particular example, synchronized DC operation means that the receive timing difference (Δt) between the signals received at the UE from the subframe boundaries of the CCs belonging to the MCG and the SCG are within a certain threshold, e.g. ±33 μs.
As used herein, unsynchronized DC operation means that the UE can perform DC operation regardless of the receive timing difference (Δt) between the signals received at the UE from the CCs belonging to the MCG and the SCG, i.e. for any value of Δt. As a particular example, unsynchronized DC operation means that the receive timing difference (Δt) between the signals received at the UE from the subframe boundaries of the CCs belonging to the MCG and the SCG can be any value, e.g. more than ±33 μs, any value up to ±0.5 milliseconds (ms), etc.
The maximum receive timing difference (Δt) at the UE consists of the following main components:                (1) Relative propagation delay, which is expressed as the difference of propagation delay between the MeNB and the SeNB, and        (2) Transmit (Tx) timing difference due to synchronization levels between antenna connectors of the MeNB and the SeNB.        
In operation, the UE signals its capability to a network node (e.g., the MeNB) indicating whether the UE is capable of synchronized and/or unsynchronized dual connectivity operation. The capability information is associated with each band or band combination supported by the UE for DC operation, e.g. the UE may indicate it supports synchronized and unsynchronized DC operation for frequency band combinations: band 1+band 3 and band 7+band 8, respectively. Based on this received UE capability information, the network node can determine whether the UE should be configured in synchronized or unsynchronized DC operation for a particular band combination.
The UE capable of CA is required to handle a maximum received time difference of signals from non-co-located serving cells (e.g., PCell and SCell) of 30.26 μs. The value of 30.26 μs corresponds to signal propagation distance of just over nine kilometers (km) considering the speed of light in free space is 3×108 m/s. In dense urban scenarios, maximum time misalignment due to multipath propagation delay is around 10 μs. This misalignment is linearly related to relative physical distance between the network nodes serving the cells from which the received time difference of signals is measured by the UE. So, there is a large amount of timing misalignment margin that may not be caused only due to distance between the network nodes. This means that there is a possibility to allow the UE to handle maximum received time difference of signals larger than 30.26 μs. For example, this can be relaxed by certain transmit timing misalignment between the network nodes (i.e., synchronization accuracy between the MeNB and the SeNB), e.g. 3 μs. Three microseconds is chosen here due to the fact that co-channel synchronization accuracy requirement for Time Division Duplexing (TDD) systems is 3 μs (which means that the tightest requirement that can be achieved is 3 μs).
The synchronized DC case essentially means that the MeNB and the SeNB transmit timing needs to be synchronized up to a certain level of time accuracy, while the unsynchronized DC case provides a random value for synchronization accuracy (i.e., anything up to 1 ms), which is higher than the accuracy required in the synchronized DC case. It is worth noting here that the received time difference, which we refer to here as Δt, is the received timing misalignment between two received signals from subframe boundaries of the MeNB and the SeNB at the UE. In other words, the receive timing difference (Δt) is not the transmit timing mismatch levels between the MeNB and the SeNB.
As the baseline option, since dual Transmitter (Tx)/Receive (Rx) is assumed, as is non-ideal backhaul, it is reasonable to assume that the MeNB and the SeNB are not synchronized to each other. The dual Tx/Rx means that the UE has one Tx/Rx pair for operation with each network node. Dual Tx/Rx means that there will potentially be separate power amplifiers for separate links, thus no strict synchronization requirement is needed. This is Case (2) in FIG. 1. FIG. 1 is a schematic diagram illustrating the Maximum Receive Timing Difference (MRTD) (Δt) at the UE. If the requirements are defined for the unsynchronized DC case, then the UE can also operate and meet requirements for the synchronized DC case. However, Case (1) in FIG. 1, which illustrates the synchronized DC case, means defining a certain synchronization accuracy between the MeNB and the SeNB applicable only for Case (2).
The receive timing difference (Δt) of radio signals from the MeNB and the SeNB may also incorporate additional delay introduced by the multipaths on individual links due to the characteristics of the radio environment. For example, in a typical urban environment, the delay spread of multiple paths received at the UE may typically be in the order of 1-3 μs. However, in wide areas like those in suburban or rural deployment scenarios, the channel delay spread due to the multipath effect of the signals observed at the UE is relatively higher, e.g. more than 1-3 μs.
In general, network-wide synchronization is not needed for DC since DC is a UE specific operation. A certain UE is connected to two eNBs in DC operation and, therefore, the synchronization requirement (whether it be a tight synchronization requirement for synchronized DC operation or a loose synchronization requirement for unsynchronized DC operation) is needed between only two eNBs (i.e., the involved MeNB and SeNB) when they serve the UE for synchronized DC operation. It should also be noted that the same MeNB and SeNB may also be serving UEs not in DC. Thus, no synchronization requirements are specified even between the MeNB and the SeNB. However, to ensure that the UE operating in DC operation is able to receive signals from the MeNB and the SeNB within the maximum allowed received timing difference (Δt), the UE needs to meet the requirements (e.g., measurement requirements, measurement accuracy requirements, Radio Link Monitoring (RLM) requirements, UE performance requirements, UE demodulation and Channel State Information (CSI) requirements, etc.):                (1) The received time difference at the UE from the MeNB and the SeNB is within the allowed limit; and        (2) The maximum transmit time difference between the MeNB and the SeNB is within a certain time limit.        
LTE has a number of power saving mechanisms, some of which are mentioned below:                (1) Discontinuous Reception (DRX);        (2) Discontinuous Transmission (DTX): the DRX equivalent at the UE transmitter;        (3) Both DRX and DTX reduce transceiver duty cycle while in Radio Resource Control (RRC) connected state; and        (4) DRX also applies to the RRC_IDLE state with a typically longer cycle time than RRC connected state.        
The usage of DRX is shown in FIG. 2, which illustrates a DRX cycle. As seen from FIG. 2, a UE monitors the Physical Downlink Control Channel (PDCCH) during the DRX ON duration of the DRX cycle. While in DRX mode (i.e., while the UE is in the DRX OFF state), the UE remains in power saving mode.
With respect to DRX, RRC sets the DRX cycle where the UE is operational for a certain period of time when all the scheduling and paging information is transmitted. This period of time is referred to as the DRX ON duration. During the DRX ON duration, the UE is referred to herein as being in the DRX ON state, or simply the ON state. During another period of time in the DRX cycle, the eNB knows that the UE is completely turned off and is not able to receive anything. This period of time is referred to as the DRX time or DRX OFF duration. Further, during the DRX time, the UE is referred to herein as being in the DRX OFF state, or simply the OFF state. Except when in DRX, the UE radio must be active to monitor PDCCH (e.g., to identify downlink data). During DRX, the UE radio can be turned off, and the eNB will not schedule the UE as it knows that the UE radio is not active. The DRX ON duration is defined by an onDurationTimer and, as such, the DRX ON duration is sometimes referred to herein as onDurationTimer. The onDurationTimer specifies the number of consecutive PDCCH subframe(s) at the beginning of a DRX cycle during which the UE is to be in the ON state in order to monitor for a PDCCH transmission.
The DRX/DTX functionality is an effective way to reduce the UE's battery power usage, but at the same time introduces further constraints in the scheduler's tasks. The immediate consequence of DRX/DTX is an average increase of packet delivery delays. Short DRX/DTX represents a further attempt to exploit the inactivity periods of the UE to save even more power. This further saving could be remarkable with certain types of traffic, but can also be very limited with others, like Voice over Internet Protocol (VoIP).
As mentioned earlier, DRX is configured by RRC mechanisms. DRX may have long or short DRX (i.e., DRX OFF) durations. The transition between long DRX and short DRX is determined by the eNB (Medium Access Control (MAC) commands) or by the UE based on an activity timer.
The application of long or short DRX largely depends on the application. A lower duty cycle could be used during a pause in speaking during a VoIP call. When speaking resumes, this results in lower latency. The proposed mechanism to avoid interruptions is applicable to either short or long DRX.
Similarly, for more non-real time services, e.g. data communication, for packets arriving at a lower rate than voice services, the UE can be off for a longer period of time. For packets arriving more often, the DRX interval (i.e., the DRX OFF duration) is reduced during this period.
Typically, all UEs are in DRX and the DRX ON duration can be as small as 1 ms. There is common DRX cycle for the PCell and the SCell(s) in CA. That means both PCell and SCell reception times will occur within the DRX ON duration. Alternatively, the network has to adapt the DRX ON duration.
When UE is configured with DRX, the UE performs intra-frequency, inter-frequency, and inter-Radio Access Technology (RAT) measurements according to the DRX cycle, e.g. typically once per DRX cycle especially for a DRX cycle of 40 ms or longer. Therefore, the measurement time is a function of DRX cycle length, i.e. scales with the DRX cycle length of configured DRX cycle.
Interruptions may occur at the UE to any carrier when one or two other carriers are configured, de-configured, activated, or deactivated. There can be many types of interruptions, which are listed below:                Interruptions at SCell addition/release for intra-band CA;        Interruptions at SCell addition/release for inter-band CA;        Interruptions at SCell activation/deactivation for intra-band CA;        Interruptions at SCell activation/deactivation for inter-band CA;        Interruptions during measurements on the Secondary CC (SCC) for intra-band CA;        Interruptions during measurements on the SCC for inter-band CA;        Interruptions at SCell addition/release with multiple downlink SCells;        Interruptions at SCell activation/deactivation with multiple downlink SCells;        Interruptions during measurements on the SCC with multiple downlink SCells; and        Interruptions at overlapping addition/release/activation/deactivation of SCells.        
In addition to these interruptions as experienced in legacy CA systems, interruptions may be required for many different reasons in a UE supporting DC. For example, the interruptions may include:                Interruptions on the PCell or SCells due to configuration of pSCell or another SCell or vice versa;        Interruptions on PSCell or SCells due to configuration of measurement gap length in MCG; and        Interruption on any carrier due to DRX ON duration of another carrier.        
Systems and methods for avoiding or reducing the number of interruptions at a UE operating in a DC mode of operation are desired.